1. Field of the Invention
This invention pertains to wire welding, and more particularly to apparatus that conducts welding power to a weld wire.
2. Description of the Prior Art
To properly perform wire welding, it is imperative that the conduction of welding power from a welding gun to the weld wire be controlled. Historically, welding power conduction occurred inside a copper contact tip in the gun. The contact tip had a hole through which the weld wire passed. The relatively long length of the contact tip and a small clearance between the contact tip hole and the weld wire increased the probability of smooth and consistent power conduction.
However, it has long been recognized that a problem existed in conducting the welding power from the contact tip to the weld wire. The problem had several aspects. The first was the lack of constant contact between the contact tip and the weld wire. Under the great majority of operating conditions, the weld wire was indeed in contact with the contact tip. Nevertheless, in some instances there were momentary conditions in which no contact occurred. In those situations, the welding current could arc across the clearance between the contact tip and the weld wire. The arcing was detrimental to the welding process.
Some prior welding machines relied on the inherent cast of the weld wire to produce a constant contact between the contact tip and the weld wire. However, non-uniformities of the weld wire cast, combined with movements in space of the welding gun during operation, inevitably resulted in momentary breakages of contact between the contact tip and the weld wire.
A second aspect of the welding power conduction problem concerned the size of the area of the interface between the contact tip and the weld wire. As mentioned, for the great majority of time during a welding process, the weld wire was in contact with the contact tip. However, even during those times of contact the areas of contact were usually very small. The typically large welding currents resulted in very high current densities, which tended to produce tiny microwelds between the contact tip and the weld wire. The small welded areas between the contact tip and weld wire were usually quickly remelted or mechanically broken. However, a stick-slip type of weld wire feeding resulted. The stick-slip feeding of the weld wire caused by repeated microweld creation and breakage contributed to poor welding performance.
A third aspect of the problem involving welding power conduction in prior welding guns was that is was possible, and even likely, that at some times during a welding operation the weld wire contacted the contact tip at more than one interface. At any particular moment, the locations of the interfaces were randomly variable along the length of the contact tip. Because of the greater electrical resistance of the weld wire than the contact tip, the amount of welding power conduction was greatest at the interface that was closest to the downstream end of the contact tip. As a result, almost all of the preheating of the weld wire occurred between the downstream interface and the workpiece. Since the interface furthest downstream invariably changed location within the contact tip during a welding operation, the amount of preheating of the weld wire also varied. Changes in the weld wire preheating changed the characteristics of the welding arc and therefore were potentially detrimental to satisfactory welding.
Further aspects of the problems involving prior contact tips included wear of the tip clearance hole. The wear contributed to random and inconsistent interfaces between the contact tip and the weld wire. The weld wire frequently had shavings and other debris clinging to it. When the foreign matter entered the contact tip it could become trapped and prevent the weld wire from feeding properly.
Related problems included spatter from the welding arc that could fly up and stick to the end of the contact tip, where it was likely to build up and stick to the weld wire. Burnback caused by a failure anywhere in the welding machine could destroy any contact tip. Similarly, excessive heat from the welding arc could cause a contact tip to reach a plastic state and wear out prematurely.
It is therefore well recognized that a contact tip can fail in a number of ways. For example, U.S. Pat. No. 3,112,393 mentions the tendency of a weld wire to arc and fuse to the contact tip, which results in a short life for the contact tip.
Prior contact tips were often treated as low cost consumable items. Yet, a contact tip failure could disrupt an entire production line. In addition, a malfunctioning contact tip caused inconsistent or improper operation of other components of a welding machine such as the weld wire feeder or the gas flow. Frequently, other welding machine components than the contact tip were searched and mis-diagnosed as the source of the problem.
Accordingly, numerous attempts have been made to solve one or more aspects of the contact tip and the weld wire interface problem. For example, to assure a constant interface, power carrying wire feed rollers were used in some welding machine guns to ensure a constant location of the interface. The feed rollers were not the complete answer, however, because additional interfaces were present at the contact tip. As mentioned, the electrical resistance of the contact tip was less than the resistance of the weld wire. Hence, even with conductive feed rollers the most downstream interface between the contact tip and the weld wire remained random and uncontrollable.
Other proposed solutions included a non-circular clearance hole in the contact tip, such as is shown in U.S. Pat. No. 5,635,091. U.S. Pat. No. 5,278,392 shows a contact tip that is covered with a graphite cover. U.S. Pat. Nos. 4,945,208 and 4,978,831 teach a chrome plated bushing covering the downstream end of a contact tip to eliminate carbon residue on the tip.
A solution for arcing and microwelding proposed by U.S. Pat. No. 3,112,393 is a contact tip having a helical clearance hole for the weld wire. A somewhat similar proposal is described in Great Britain patent 2,074,069A. That patent shows a contact tip with one or two projections inside the contact tip clearance hole. The projections force the weld wire to change direction inside the contact tip and thereby assure good contact with the contact tip. Another helix-related contact tip is described in U.S. Pat. No. 4,733,052, in which a contact tip is in the form of a rectangular strip wound into a helix. A central longitudinal opening through the helix is smaller than the diameter of the weld wire, such that the weld wire is resiliently squeezed by the contact tip.
U.S. Pat. No. 4,563,569 describes a welding gun in which a weld wire guide tube is transversely pivotable within the gun body. Springs within the gun urge the guide tube transversely such that the weld wire is urged into transverse contact with the clearance hole in the contact tube, which is fixed relative to the gun body.
In U.S. Pat. No. 3,102,947, arcing between the contact tip and the weld wire is prevented by lining the contact tip with an insulative sleeve. Welding power is transferred to the weld wire upstream of the contact tip.
U.S. Pat. No. 4,988,846 describes a contact tip with an arcuate groove cut in the wall between two coaxial clearance holes for the weld wire. The weld wire contacts and is deflected by the groove surface, thus assuring an intimate interface between the contact tip and the weld wire. The same principal is used in U.S. Pat. No. 3,142,746, which discloses a ball that partially closes the clearance hole in the contact tip. The ball is biased radially against the weld wire to assure a constant interface between the contact tip and the weld wire.
U.S. Pat. No. 4,731,518 teaches a welding gun with a reverse radius in the head tube between the gun handle and the contact tip. The reverse radius causes the weld wire to be biased against the clearance hole in the contact tip.
Japan patent JP-11170052-A describes three parallel cylindrical columns arranged in a triangle to form a central hole through which a weld wire passes. The three columns are held in place against each other by an elastic band surrounding the columns. The elastic band, in turn, is captured inside a tube. The elastic band urges the columns radially inwardly against the weld wire.
Despite the long felt need for a controlled and reliable interface between a contact tip and a weld wire, and despite the numerous attempts to solve the interface problem, none of the prior solutions is entirely satisfactory.
In accordance with the present invention, an alternate current path for a metal inert gas (mig) gun reliably assures a stable interface between a contact and a weld wire. This is accomplished by apparatus that includes fingers that maintain positive contact with the weld wire under all operating conditions.
According to one aspect of the invention, the fingers are an integral part of a conductive insert that fits in a welding gun diffuser. The fingers are on an end of a tubular body that is split longitudinally. The body has a nominal outer diameter that is slightly greater than an associated inner diameter of the diffuser. Installing the insert in the diffuser causes the body and fingers to collapse radially slightly. The fingers are then spaced apart a distance less than the diameter of the weld wire. Feeding the weld wire resiliently springs the fingers radially apart. The natural inwardly restoring force of the insert material produces a reliable interface for conducting welding power from the diffuser to the weld wire. The insert thus provides additional stable paths for the welding power from the welding gun to the weld wire. The current densities along the additional paths are reduced. In turn, the reduced current densities reduce the likelihood of microwelds occurring at the weld wire that could cause stick-slip feeding during welding operations.
According to another aspect of the invention, the fingers are incorporated into a contact tip that is part of a contact tip assembly. A body on a first end of the contact tip is threaded to engage a diffuser. The fingers are at the second end of the contact tip and are integral with the body. The outer diameter of the fingers at the contact tip second end define concentric partial frusto-conical surfaces that diverge toward the second end. An annular member such as a sleeve fits over the contact tip. A frusto-conical surface on the inner diameter of the sleeve matches the frusto-conical surfaces of the fingers. A spring is interposed between the sleeve and the diffuser. The spring urges the sleeve frusto-conical surface against the finger frusto-conical surfaces and thereby biases the fingers radially inwardly against the weld wire, which is fed through the contact tip and the fingers. Not only do the fingers provide a reliable interface that eliminates arcing, but they also assure that the interface is only at the downstream end of the contact tip. Consequently, there is only one consistent location for the interface. Weld wire preheating is thus constant, which greatly aides in controlling the welding process. To be certain that the weld wire does not protrude through any of the spaces between the fingers, a liner may be assembled inside the contact tip surrounding the weld wire.
It is a feature of the invention that the contact tip assembly further comprises a non-metallic shield surrounding the contact tip and the sleeve. The weld wire passes through a clearance hole in an end wall of the shield. The shield protects the contact tip from welding spatter and burnback. To prevent burnback at the contact tip, the shield clearance hole is substantially larger in diameter than the weld wire diameter. Any molten material created during a burnback situation is not sufficient to plug the shield hole and thereby cause harm to the contact tip.
In a modified embodiment of the invention, the contact tip assembly has a contact tip with fingers that define concentric partial frusto-conical surfaces that converge toward the second end. An annular member in the form of a casing has a first end with a frusto-conical inner surface that matches the contact tip frusto-conical surfaces. A spring acts between the casing second end and contact tip to urge the contact tip frusto-conical surfaces against the casing frusto-conical surface. The contact tip fingers are thus biased radially inwardly against a weld wire. The radially inward force of the fingers on the weld wire eliminates arcing and also provides a reliable interface between the contact tip and the weld wire only at the downstream end of the contact tip.
An alternate embodiment of the alternate current path according to the invention comprises a contact tip assembly with a pivotable finger incorporated into a contact tip. One end of the contact tip engages a diffuser. The outer diameter of the contact tip is recessed at its second end such that a short length of the weld wire is exposed. The finger is connected to the contact tip for pivoting about an axis perpendicular to the weld wire. The finger has a tab that fits in the contact tip recess and contacts the exposed length of the weld wire. A spring acts between the contact tip and the finger second end in a manner that presses the finger tab against the weld wire. The result is a reliable interface between the weld wire and the contact tip only at the contact tip downstream end.
In a further alternate embodiment, the alternate current path comprises a contact tip assembly having a finger that is wedged in a contact tip. The outer diameter of the contact tip is slotted such that a length of the weld wire is exposed. The contact tip is further shaped with an angled notch adjacent the recess. The finger fits in the contact tip slot and is held there by an end that fits in the angled notch. The finger has a longitudinal surface that contacts the exposed length of the weld wire to assure a single reliable interface between the weld wire and the contact tip.
Other advantages, benefits, and features of the present invention will become apparent to those skilled in the art upon reading the detailed description of the invention.